Hello Rayniero.
A TCP checksum is used to determine if a TCP segment has been transmitted successfully and without corruption. The sender of the segment computes a checksum by applying an algorithm to the payload and getting a result. The result is placed in the TCP checksum field. When the segment reaches the receiver, the checksum is recomputed with the same algorithm and compared to the checksum sent by the sender. If a bit is flipped or some other badness happens to the segment in transit, then it is highly likely that the receiver of that broken packet will notice the problem due to a checksum mismatch. This provides end-to-end assurance that the data stream is correct. If the checksum does not match, the segment is dropped. The “reliability” characteristics of TCP kick in and the bad segment is requested again from the sender.
I hope this was helpful!
Laz